Which of the following is not digested by humans?

The process by which the body breaks down food into smaller, absorbable components and converts them into energy and nutrients is known as digestion.

Cellulose is a complex carbohydrate found in plant cell walls that is not digestible by humans. Although indigestible, cellulose plays an important role in digestive system health by adding bulk and promoting regular bowel movements. 

Cellulase, an enzyme required to break down cellulose into simpler sugars that can be absorbed and used for energy, is absent in humans.

Carbohydrates, proteins, and fats are digested in the intestine and broken down into sugars, amino acids and glycerol respectively.